Lithium-ion batteries utilize carbon anodes, lithiated transition metal oxide cathodes, and an organic-based solvent electrolyte with a dissolved conducting salt such as lithium hexafluorophosphate (LiPF6). These batteries currently dominate the battery market in the area of cellular phones, cam-recorders, computers, and other electronic equipment. However, attempts to apply these battery technologies to electric and hybrid vehicles have met with limited success. Problematic areas include safety, calendar life, cost, and, in the case of hybrid vehicles, high rate capability for power assist and regenerative braking.
Lithium-manganese-oxide-spinel-based electrodes have drawn enormous attention as a cathode material, since the Mn is less expensive compared to the Co and Ni that are currently being used in the commercial Li-ion cells. Mn also has far better power, is safer and is environmentally benign. However, the poor capacity retention (fading) of LiMn2O4 spinel has been a major drawback of this technology and has prevented its wide acceptance by manufacturers for commercialization. Moreover, the deterioration of its electrochemical performance, including capacity loss, impedance rise, and material instability, is far more severe at higher temperatures, above 40-50° C., that can easily be reached in portable electronic devices or hybrid electric vehicles. Although several factors have been reported to be responsible for the electrochemical degradation of the spinel based cells, it is generally attributed to the instability of manganese spinel. This degradation likely results from the formation and dissolution of manganese ions in the organic based electrolyte.
The dissolution of the manganese originates from the instability of the manganese (III) ions on the surface of the manganese spinel electrode during cycling in the LiPF6 based organic electrolyte that is used in nearly all commercial Li-ion batteries today. The manganese (III) instability results from the disproportionation reaction that occurs on the surface of the spinel electrode (2Mn3+(stable solid)→Mn4+(stable solid)+Mn2+(unstable solid, tending to be dissolved)). The Mn2+ ions that are formed dissolve in the LiPF6-containing organic electrolyte. Thereafter, the dissolved manganese ions diffuse through the electrolyte to the graphite anode where they are likely reduced to manganese metal and deposited on the anode surface. This phenomenon results in a huge increase in the impedance of the anode and a loss of active lithium from the cell, as well as the degradation of the spinel cathode. The result is a cell with poor electrochemical performance and little or no power.
In addition, manganese dissolution has been attributed to acid attack, and occurs even with trace levels of HF, commonly present in LiPF6-based electrolytes. Together with the manganese ion diffusion problem as mentioned above, the presence of acid such as HF causes formation of a partially protonated λ-MnO2 phase. This phase is not totally electrochemically active, since the protons are bonded to octahedral oxygen sites of the cubic close-packed oxygen array of MnO6. This scenario suggests that with the manganese dissolution there is also the partial protonation of the λ-MnO2 that leads to the deterioration of manganese spinel cathode material.
Since its introduction by Padhi et al. [A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc., 144 (4), 1188 (1997)], LiFePO4 olivine material has become one of the most studied cathodes for lithium-ion battery (LIB) applications. Unlike many cathodes, the electrochemistry of this material involves the Fe2+/Fe3+ redox couple, which occurs at a voltage of 3.45 V and has a theoretical capacity of 170 mAh/g. Discharged and charged positive active materials, LiFePO4 and FePO4, respectively, have the same structural arrangement, i.e. same space group and close crystalline parameters, leading to very good system stability during the electrochemical cycling process. This stability is not altered by Fe3+ ion generation, in contrast to the highly oxidizing Ni4+ ions that are involved in the charging of LiMIIIO2 (M=Ni, Co) layered material. In addition, the cutoff voltage of 3.45 V is low enough to prevent the acceleration of electrolyte aging but not so low as to sacrifice the energy density or electrochemical performance of the olivine. Moreover, LiFePO4 is an inexpensive material, non toxic and environmentally benign. For these reasons, LiFePO4 has been considered as a potentially attractive cathode material for LIB.
However, LiFePO4 is an insulating material, which seriously limits its rate capability and thus its calendar life. Although extensive work has been conducted recently to enhance the electronic conductivity of the material, much room for improvement exists.
To prevent degradation of the cathode material, several approaches have been attempted, including cationic substitution of manganese or surface modification (coatings) of the spinel cathode or of graphite anode surfaces. See, e.g., C. Sigala, A. et al., J. Electrochem. Soc., 148, A826 (2001).; I. J. Davidson, et al., J. Power Sources, 54, 205 (1995); M. Yoshio, et al., J. Power Sources, 101, 79 (2001); and A. M. Kannan and A. Manthiram, Electrochem. Solid State Lett., 5, A167 (2002). While these methods have shown some promise at room temperature, none have prevented significant electrochemical deterioration due to the manganese dissolution at elevated temperatures. See, e.g., A. Blyr, et al., J. Electrochem. Soc., 145, 194 (1998); and G. G. Amatucci, et al., J. Electrochem. Soc., 148, A171 (2001). Accordingly, there is a need in the art to develop electrolyte systems that protect the cathode surface from any unwanted reactions. Furthermore, there is a need in the art for batteries using such electrolyte systems.